I have the same experience with Backblaze. 3 years ago I tried to restore my files from Backblaze, using their desktop client.

First thing I noticed is that if it can't download a file due to network or some other problem then it just skips it. But you can force it to retry by modifying its job file which is just an SQLite DB. Also it stores and downloads files by splitting them into small chunks. It stores checksums of these chunks, but it doesn't store the complete checksum of the file, so judging by how badly the client is written I can't be sure that restored files are not corrupted after the stitching.

Then I found out that it can't download some files even after dozens of retries because it seems they are corrupted on Backblaze side.

But the most jarring issue for me is that it mangled all non-ascii filenames. They are stored as UTF-8 in the DB, but the client saves them as Windows-1252 or something. So I ended up with hundreds of gigabytes of files with names like фикац, and I can't just re-encode these names back, because some characters were dropped during the process.

I wanted to write a script that forces Backblaze Client to redownload files, logs all files that can't be restored, fixes the broken names and splits restored files back into chunks to validate their checksums against the SQLite DB, but it was too big of a task for me, so I just procrastinated for 3 years, while keeping paying monthly Backblaze fees because it's sad to let go of my data.

I wonder if they fixed their client since then.